by Wei Li
figures by Catherine Ding

The universe is massive, with an estimated 70 quintillion planets—that is 70 followed by an additional 18 zeros. In the Milky Way alone, where we reside, there are billions of planets. With these huge numbers, Earth seems very insignificant in the grand scheme of things. This raises the question: are we truly alone in this vast space of the universe? Surely, there must be another planet out there with life, right?

This is a question that organizations such as the National Aeronautics and Space Administration (NASA) have been trying to answer for decades. Using telescopes like the Hubble and the Webb, NASA can probe planets for their potential habitability. However, as powerful as these telescopes are, they cannot be used to detect the presence of extraterrestrial life. And, while there have been many hints of potentially habitable planets, as far as we know, Earth is the only rock in this large universe that holds life.

What is the key to finding habitable planets?

How does NASA go about looking for planets that may be potentially habitable? The key is as simple as it is important: water.

Indeed, in order to understand how a planet can be habitable, scientists turn to Earth, our one and only example of a planet that has successfully sustained life. By analyzing characteristics of Earth such as the atmosphere, chemical composition, and even its evolutionary history, we can compare Earth to other planets and deduce whether or not a planet could sustain life.

All living organisms require water to survive, including humans, with about 70% of the human body being made up of water. Interestingly, as if paralleling the human body, about 70% of Earth is also covered in water. It is therefore no wonder that the presence of water is something NASA looks for in their search of habitable planets. Their exploration of our neighboring planets, Mars and Venus, has shown that both planets once harbored water, so what happened? Where did all the water go? 

By understanding what happened to our neighbors, we can better understand what conditions are needed for a planet to be habitable, as well as better appreciate just how unique of a situation Earth is in (Figure 1).

Venus: Too slow and too close to the sun

In our solar system, Venus is the most similar to Earth with respect to mass and size, and it is also the planet physically closest to Earth. Theoretically, Venus should be as habitable as Earth, right? 

A recent study led by a group of astrophysicists at the University of Geneva and the National Centre of Competence in Research PlanetS in Switzerland did a climate simulation of both Earth and Venus showing that, unfortunately, Venus suffered a fate that Earth did not: being slightly too close to the sun.

At the very beginning of a planet’s formation, the surface of the planet is molten, and any water is present as steam. This resembles a pressure cooker scenario, where water vapor traps the sun’s heat, making it extremely hard for the planet to cool down enough for the water vapor to condense into oceans (Figure 2). Thankfully, Earth is far enough from the sun that the surface cooled during night time, causing water vapor to condense and oceans to form. That is not the case for Venus, however. The scientists in the study calculated the heat that resulted from the steam on Venus, and while it may be only slightly more than that on Earth, it was just enough that the steam never condensed to become oceans and simply evaporated off the planet. 

Therefore, in addition to the presence of water, scientists are now assessing another important factor: the planet has to be in a habitable zone (Figure 1). In other words, the planet needs to be orbiting around its respective sun at the right distance: not too close to the sun like Venus and not too far where any liquids would turn to ice. 

Another possible reason for Venus’ inability to form oceans may be because it spins too slowly. Standing at the equator of Earth, you will be flying through space at 1,040 mph, while on Venus, you would only be going at 4 mph. Another climate simulation study conducted by NASA showed that the slow spin of Venus allowed water vapor to form clouds earlier than Earth. These clouds increased the amount of water vapor in Venus’ atmosphere, trapping the sun’s heat even more. Scientists believe that increasing the spin rate of Venus by about 20 times might have cooled Venus enough to save it from its current ocean-less state.

Mars: Too small to hold onto water

Mars is our other neighbor. In the early days of Mars exploration, NASA found ample evidence of water on Mars. Using rovers and analyzing collected sediments on the surface of Mars, scientists deduced that Mars was once wet and covered with plenty of rivers, lakes and oceans, just like Earth. This means that, approximately 3.5 billion years ago, Mars had a similar climate to Earth. But today, Mars is a dry desert and unfit to sustain life. How did this planet, a neighbor to Earth, change from a potentially habitable planet to one with conditions too harsh for life?

A recent study suggests an answer to that question: Mars may be too small to hold onto sufficient amounts of water. Mars is a lot smaller than our home planet—only slightly more than half the size of Earth. The scientists found that larger planets, like Earth, have stronger gravitational pulls and thus can more easily hold evaporated molecules like water, while smaller bodies like Mars cannot.

Earth: A unique situation

Despite all we know so far, there is also a lot more to learn, which is why NASA still has rovers on the surface of Mars and satellite flybys to observe Venus. There is a new satellite flyby planned for Jupiter and its 79 moons in June 2022 in order to understand the atmosphere of Jupiter and look for signs of life. While these tools are great, they are not yet powerful enough to search through the billions of planets that we know exist. With ongoing explorations and analyses, maybe one day we’ll find another planet with life like Earth.

Perhaps, though, Earth really is a one of a kind situation, and there are more uninhabitable planets than actual habitable planets out there. Even as we continue on our quest to find other planets with life, we need to recognize that Earth may be more unique and precious than we think, and that we definitely need to start cherishing it more.

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Wei Li is a fourth-year Ph.D. student in the Chemistry and Chemical Biology program at Harvard University.

Catherine (Xiaoxiao) Ding is a fourth-year Applied Math Ph.D. student in the School of Engineering and Applied Sciences at Harvard University where she is studying programmable materials.

For more information:

• Check out NASA’s Mars Exploration Program.
• To learn additional information about exoplanets and life in space, read more here.